1. Field of the Invention
The present invention relates to a three way electromagnetic valve whereby a passage can be switched by the displacement of a ball driven by an electromagnetic device.
2. Description of Related Art
FIG. 14 is a sectional view of a conventional normally closed three way electromagnetic valve disclosed in, for example, Japanese Laid-Open Patent Application No. 8-105563. In the drawing, a bobbin 1, which is a resin molded article, is provided with an input port 2, an output port 3, a discharge port 4, and a fluid passage 1A which communicates the ports 2 through 4 and which extends in the axial direction. The bobbin 1 also has a plate 5 and terminals 6, 7 which constitute a magnetic passage and which are formed by insert molding. Provided on the outer periphery of the bobbin 1 are a coil winding portion 1a and O ring grooves 1b, 1c.
A coil 8 is fixed onto the coil winding portion 1a, and O rings 9, 10 for preventing the leakage of fluid are respectively fitted in the O ring grooves 1b, 1c. The winding start tip of the coil 8 is wound onto the terminal 6 and soldered thereto, while the winding end tip thereof is wound onto the terminal 7 and soldered thereto.
The fluid passage 1 A is provided with a tapered discharge valve seat 1d and an input valve seat 1e; a sealing ball 11 which moves into contact with or away from the input valve seat 1e is inserted in the fluid passage 1 A at the input port 2 end. A retainer 12 for preventing the ball 11 from coming off is inserted in the input port 2 and fixed by a thermally caulked section 1f.
In the bobbin 1, a plunger 13 made of a magnetic material is slidably inserted in the axial direction. FIG. 15 is a sectional view of the plunger 13 of FIG. 14; the plunger 13 is equipped with a push rod 13a for pushing the ball 11, and a plunger seat 13b which moves into contact with or away from the discharge valve seat 1d.
The portion of the bobbin 1 where the coil 8 is attached is enclosed by a cylindrical case 14 to which a flange 15 has been welded to ground the case 14. An end of the terminal 7 is welded to the case 14. A bush 16 made of a magnetic material is inserted in the rear end of the bobbin 1 in such a manner that it does not come in contact with the plunger 13. The bush 16 is magnetically connected to the case 14 via a holder 17 made of a magnetic material. The bush 16 and the holder 17 are fixed with respect to the bobbin 1 by a plate 18 made of a nonmagnetic material. The terminal 6 is protected by a cover 19 against external forces.
The operation of the three way electromagnetic valve will now be described. When the coil 8 is not in an excited state, that is, when it is in a de-energized state, the pressure of a fluid applied to the input port 2 pushes the ball 11 against the input valve seat 1e to cut off the flow of the fluid coming through the input port 2. At this time, the plunger 13 is pushed up by the ball 1, and the plunger seat 13b is released from the discharge valve seat 1d, causing the output port 3 and the discharge port 4 to be communicated.
The moment the coil 8 is excited, magnetism runs through a magnetic circuit constituted by the case 14, the plate 5, the plunger 13, the bush 16, and the holder 17, causing the plunger 13 to be attracted to the plate 5. The attracting force causes the plunger 13 to move toward the plate 5 against the pressure of the fluid acting on the ball 11. This pushes the plunger seat 13b against the discharge valve seat 1d so as to cut off the flow of the fluid to the discharge port 4, and the push rod 13a pushes the ball 11 so as to release it from the input valve seat 1e, thus communicating the input port 2 with the output port 3.
FIG. 16 is a sectional view illustrative of an example of a conventional normally open three way electromagnetic valve. In the drawing, a bobbin 21 which is a resin molded article is provided with an input port 22, an output port 23, a discharge port 24, and a fluid passage 21A which communicates the ports 22 through 24 and which extends in the axial direction. The bobbin 21 also has a yoke 25 and terminals 26, 27 which constitute a magnetic passage and which have been formed by insert molding. The yoke 25 has been machined to have a flanged cylindrical shape by drawing a discoid iron plate. Provided on the outer periphery of the bobbin 21 are a coil winding portion 21a and O ring grooves 21b, 21c.
A coil 28 is fixed onto the coil winding portion 21a, and O rings 29, 30 for preventing the leakage of fluid are respectively fitted in the O ring grooves 21b, 21c. The winding start tip of the coil 28 is wound onto the terminal 26 and soldered thereto, while the winding end tip of the coil 28 is wound onto the terminal 27 and soldered thereto.
The fluid passage 21 A is provided with a tapered discharge valve seat 21d and an input valve seat 21e; a sealing ball 31 which moves into contact with or away from the input valve seat 21e is inserted in the fluid passage 21A at the input port 22 end. A retainer 32 for preventing the ball 31 from coming off is inserted in the input port 22 and fixed by a thermally caulked section 21f.
In the bobbin 21, a plunger 33 made of a magnetic material is slidably inserted in the axial direction. FIG. 17 is a sectional view of the plunger 33 of FIG. 16. The plunger 33 is equipped with a push rod 33a for pushing the ball 31, a plunger seat 33b which moves into contact with or away from the discharge valve seat 21d, and a rib 33c for shifting the flow of a fluid to the downward direction in the drawing.
The portion of the bobbin 21 where the coil 28 is attached is enclosed by a cylindrical case 34 to which a flange 35 has been welded to ground the case 34. The terminal 27 is welded to the case 34. A core 36 which has a through hole 36a and which is made of a magnetic material is press-fitted in the rear end of the bobbin 21. A spring 37 urging the plunger 33 toward the ball 31 is provided between the core 36 and the plunger 33 in the bobbin 21. A plate 38 is caulked to the rear end of the case 34. The terminal 26 is protected by a cover 39 against external forces.
The operation of this three way electromagnetic valve will now be described. When the coil 28 is not in an excited state, that is, when it is in a de-energized state, the urging force of the spring 37 moves the plunger 33 to the ball 31, causing the plunger seat 33b to be pushed against the discharge valve seat 21d, and the ball 31 is pushed by the push rod 33a so as to be released from the input valve seat 21e, thus causing the input port 22 and the output port 23 to be communicated.
The moment the coil 28 is excited, magnetism runs through a magnetic circuit constituted by the yoke 25, the plunger 33, the core 36, the plate 38, and the case 34, thus causing the plunger 33 to be attracted to the core 36 against the spring 37. This releases the plunger seat 33b from the discharge valve seat 21d and also pushes the ball 31 against the input valve seat 21e, thus causing the output port 23 and the discharge port 24 to be communicated.
In the normally closed or normally open three way electromagnetic valve configured as described above, since the push rods 13a, 33a and the balls 11, 31 are disposed in the fluid passages 1A, 21A, the sectional areas of the fluid passages 1A, 21A are inevitably small. Therefore, when oil, for example, is employed as a fluid, smooth flow of the oil is prevented when the viscosity of the oil increases at low temperature because of the small sectional areas of the fluid passages 1A, 21A.
To cope with the problem of small sectional areas, a conventional electromagnetic valve as shown, for example, in FIG. 18 and FIG. 19 increases the sectional areas of the fluid passages 1A, 21A by increasing the inside diameters of the bobbins 1, 21 in relation to the outside diameters of the balls 11, 31. This design undesirably causes the push rods 13a, 33a to shake in the radial direction when the push rods 13a, 33a push the balls 11, 31, leading to unstable operation.
FIG. 20 is a schematic representation illustrating the relationship between the plunger 13 and the ball 11 when the normally closed three way electromagnetic valve is in a de-energized state, and FIG. 21 is a front view illustrative of the position of the ball 11 of FIG. 20. FIG. 22 is a schematic representation showing an example of a state of the assembly shown in FIG. 20 wherein energization has been started; FIG. 23 is a schematic representation showing another example of the state of the assembly shown in FIG. 20 wherein energization has been started; FIG. 24 is a front view illustrating the position of the ball 11 of FIG. 22 and FIG. 23; and FIG. 25 is a schematic representation illustrating a state wherein the operation cycle of the plunger 13 of FIG. 20 has been completed. Thus, there are two ways to reach the state illustrated in FIG. 25; in one way, the state shown in FIG. 20 is changed to the state shown in FIG. 25 through the state shown in FIG. 22, while in the other way, the state shown in FIG. 20 is shifted to the one shown in FIG. 25 through the state shown in FIG. 23.
In the state illustrated in FIG. 23, the distal end of the push rod 13a has run aground on the ball 11; therefore, the plunger 13 significantly tilts and moves while being pushed against the inner wall surface of the bobbin 1. Hence, it is more difficult for the plunger 13 to move in the state shown in FIG. 23 than in the state shown in FIG. 22, thus requiring larger operating current. Thus, since the operation of the plunger 13 is unstable, the operating current and the operating voltage fluctuate accordingly.